After another one of Jeremy’s unsuccessful attempts to bring Aelita to life, the virtual, young girl’s computer programme bugs up : if she deactivates a tower, she will disappear from Lyokô once and for all ! A race against time starts for Jeremy who must de-bug Aelita’s programme before Xana’s next attack, or else !. Unfortunately, the super-computer jumps into action, sending destructive ultrasound-waves which make the school building collapse and put Odd’s life in grave danger… Ulrich and Yumi join Aelita on Lyokô, but in view of the scale of the attack, they soon realise that Jeremy will not have enough time to solve the problem before the tower must be deactivated. They have no choice but to sacrifice Aelita…

Detailed summary

Once again, Jeremy has spent the night in the lab, working on the materialisation program. He talks to his friends in the morning and produces a test tube! Inside rests a strand of hair of a very characteristic pink colour… This is a hair from Aelita…

Taking his friends to the factory, he explains to them that the hair of a human being, simple to materialise, contains all the genetic information of the person. Coincidentally, he’s succeeded in materialising a hair from his young friend, meaning that it’s possible to fully materialise her… Without waiting any longer, he launches the program, which fails due to an error. Disappointed, Jeremy dismisses his friends to do research on the origin of the problem.
On Earth, certain objects start breaking in unusual ways: for example, Yumi breaks a glass in the cafeteria as she tightens her grip on it while getting angry at Sissi…
But these events aren’t the highest thing on their list of priorities because Jeremy calls his friends, in a panic. By materialising the hair, he bugged up the AI and the materialisation failed… Even worse, now, if Aelita deactivates a Tower, she’ll be reformatted and disappear… The brainiac mopes about his error and Aelita tries to encourage him, saying that a program can be debugged…

During the day, other objects break here and there, firstly glass objects, then the ceiling collapses…nobody can figure out the reason behind the destruction.
Aelita then informs Jeremy of the activated Tower in the Forest Sector… The situation is dramatic: Aelita isn’t debugged yet and can’t deactivate the tower without risking herself…
On Earth, the school’s foundations begin to waver, the walls crack, and the buildings must be evacuated immediately. Ulrich heads to the factory, as does Yumi, but Odd enters the dorms to retrieve Kiwi.
At the same time, he warns Milly and Tamiya who hadn’t heard the evacuation announcement… The three students are too slow, a gaping crevasse forms in the hallway, trapping them on the second floor of the school…

According to Jeremy, XANA destroyed the school’s foundations using ultrasound, inaudible to the human ear. This also explains how Kiwi anticipated each wave of attack because dogs are capable of hearing ultrasound… Having just arrived in the Forest Sector, Yumi, Ulrich and Aelita are assaulted by Krabes… Losing a number of life points, but retaliating when they can, the two warriors make their way towards the Tower.

At Kadic, the floor collapses and Odd, protecting the two young girls, trying to ensure they avoid a fatal fall and hoping that the Return to the Past comes quickly…
Though the path is cleared, Aelita can’t deactivate the tower safely, which gives XANA the opportunity to send more monsters. The heroes hide in a hollow trunk. Jeremy’s attempt to debug Aelita fails and the humanoid decides to deactivate the Tower. Asking Yumi and Ulrich to escort her one last time, she apologises to Jeremy and says “Goodbye” to her sweetheart who begs in vain for her not to enter the Code…
…Lyoko.

Aelita’s sacrifice saves Odd from the complete collapse of the school… Terrified, Jeremy mechanically launches the RTTP.
In mourning, the band of heroes loses almost all hope. Yumi however finds a glimmer of it, reminding them of the famous hair. If it contains the entire digital genetic information for Aelita, maybe they can recreate the AI by using it. The company does so and it works, much to the happiness of Jeremy. Everything is back to normal…

- Return to the past to repair the damage caused by the ultrasound, and erase the memories of Sissi, Jim, Ms Hertz, Mr Delmas, the students, Milly and Tamiya.
- Jeremy materialises one hair from Aelita.

Trivia

About the characters

- First time Aelita dies.
- Mr Delmas claims to have been working for 30 years as a principal.
- Only episode in which Yumi and Ulrich spar outside of a class run by Jim.

About errors

- The red halo of the activated tower disappears in two shots.
- We see during one scene that Ulrich has Yumi’s phone, which is red…
- The factory microphone disappears from Jeremy’s left ear in one shot.
- The factory computer shows three green icons instead of two plus Aelita’s (yellow) in one shot.
- When the factory computer shows “Ulrich vs Krabe” we can see three Krabes when there’s only one.
- When the principal announces to all the students that they need to leave the buildings, we see Emilie in class with Ms Hertz and in the next shot, she’s outside.
- Odd’s pants are entirely the one shade of purple in 7 shots and they’re entirely the darker colour when Jeremy explains how he got the idea of materialising the hair and after he says what happened to Sissi.
- The mouse pad for Jeremy’s computer disappears when he explains about the bug in Aelita and when he turns the screen so Aelita can’t see what he’s doing with Kiwi.
- When Yumi says she hopes Jeremy finds the solution before XANA attacks again, it’s Ulrich’s voice we hear instead of Yumi’s.
- When Jim mentions that he’s been searching for Kiwi for ages, we’re meant to hear Kiwi barking because of the ultrasound being used again. Instead, we hear nothing, and Jim appears to suddenly think of the dog randomly while observing the damage to the ceiling.
- The dorm hallway floor keeps changing texture from flat lino to planks of wood.

About the series

- French title: D’un cheveu (this title didn’t translate well into English, so they chose something completely different in the end)
- Original English title: By a Hair's Breadth

- Second time we see Sissi’s diary.
- Jim: “The dog! I’ve been after him for a long time!”
Jim makes an allusion to episode 3 “Holiday in the Fog” when Kiwi woke him up in the middle of the night.

Quotes

Yumi: Anyone seen Jeremy this morning?
Odd: I think he spent the night in the laboratory. He was working on something really important! Hm…
Yumi, Ulrich, Odd: Aelita!

Jeremy: Hey guys! Guess what I did last night?
Odd: Let’s see now…got something to do with materialising Aelita?
Jeremy: That’s right! Say, how did you guess, Odd? Well guys, I finally did it! This time I decided tot tackle the problem in a different way. You wouldn’t believe it, but-
Ulrich: Well tell us, did you do it or not?
Jeremy: I sure did! (He takes something out of his pocket)
Ulrich (unconvinced): Hm…you materialised…a test tube?
Jeremy: No, airhead! Look inside, will you?
Odd: A hair? You only materialised one hair?
Jeremy: Yep, that’s right, Odd! To be exact, it’s a hair of…Aelita’s!

Odd: Hey, come on you guys, if we don’t materialise in a couple of minutes in the classroom, we’re not gonna last a few days!

Jeremy: The only way to find out is to start over!
Odd: No, don’t do that! You’re only a hair’s breadth away!
Yumi: Can’t you ever be serious, Odd?
Ulrich: By the way, Odd, speaking of hair, your dog sheds an awful lot of it! Most of them end up on my bed too!
Odd: What’s the big deal about two or three hairs?
Ulrich: Two or three?! With all the hair he sheds I could knit you another Kiwi!

Sissi: Hey, can I sit down?
Ulrich: No…
Sissi: You know, I’m so crazy about dogs that I’m ready to keep my mouth shut about the one Odd’s hiding in his room! That is, if you promise to be nicer to me, of course!

Ulrich: Anyway, I know what I’m in for if I get on your nerves!

Odd (to Kiwi, talking about the broken window): Did you do that?

Jim: Ulrich, clean up your bed, there’s hair all over it!

Jeremy: Stop chewing my slipper!
Aelita: …
Jeremy: Oh, sorry, Aelita! I was talking to the dog! When he’s around it’s hard to concentrate! (he sees Kiwi holding his boxers in his mouth) Aaah!
Aelita: What is it?
Jeremy: (turns the screen) Oh, nothing! Give that back to me! Give it back! That dog! He took my…my…handkerchief! (bashes head on keyboard) Oh, take him away, please!

Odd (watching Sissi behind him): I always said Sissi’s head was full of plaster!
Jeremy: I’m heading for the lab! With the dog barking and the school falling apart, I can’t work in my room!

Jim: The dog! I’ve been after him for a long time!

Yumi (in her underwear): Hi Ulrich. Um, I’m not really ready yet…

Ulrich: Jeremy! How many life points do I have?
Jeremy: Hold on, uh...70! No, 60!
Ulrich: Make up your mind, Jeremy, I gotta know!
Jeremy: I can’t do a million things at once! Dodge the lasers and you won’t have to keep count!

Ulrich: XANA’s pulled out all the stops! We’re not going to last long, Jeremy! Jeremy, do you read me?

Aelita: Are you ready for one last run?

Aelita: Goodbye, Jeremy!
Jeremy: NOOOO! AELITAAAAA!

Ulrich: Jeremy, it wasn’t your fault…
Jeremy: It was. And all that…for a strand of hair…

Aelita: Jeremy! I’m ready for my materialisation now!
All: AELITA!!!
Aelita: Jeremy? Did I miss something? Why is there water on your face?
Jeremy: I’ll tell you later, Aelita! This time we only missed…by a hair!
(All laugh)